1. Field of the Invention
The present invention relates to a magneto-optical recording method capable of performing overwriting, and the magneto-optical recording medium for use in the same.
2. Discussion of Background
Recently, magneto-optical recording media utilizing magneto-optical effects, which are used as rewritable optical recording media, have been actively studied and developed. Some of these are in fact used in practice. The magneto-optical recording media are capable of recording a large volume of information with high density, and non-contact reproducing of information, with easy access. Furthermore, the magneto-optical recording media are expected to be utilized for filing document information, as file memories for video images and still pictures, and memory devices for computers. There are still several technical problems which must be solved in order to improve the performance of the magneto-optical recording media to a level equal to or higher than that of magnetic disks. One of the most significant problems is in the overwrite technology. With current commercially available magneto-optical recording medium, when information is newly written, it is necessary to first erase an originally stored information, then write in the new information. A considerable amount of time is wasted in making this erasure. This shortcoming can be eliminated by overwrite technologies. The overwrite technologies which have been proposed can be roughly classified into a technology using magnetic-field modulation systems and a technology using a light modulation system, which includes a multi-beam system and a two-layer system.
In the magnetic-field modulation system, while keeping the light intensity constant, recording is performed by reversing the polarity of a magnetic field applied in accordance with the information to be recorded. In this system, the polarity of the magnetic field has to be reversed at high speed, so that a magnetic head of a floating type has to be employed, which makes it difficult to change the recording medium to be used therewith.
On the other hand, in the light modulation system, while keeping the intensity of the magnetic-field constant, recording is performed by enabling or disabling or turning ON or OFF the application of a laser beam or by subjecting the laser beam to an intensity modulation in accordance with the information to be recorded. The multi-beam system which belongs to the light modulation system is a pseudo-overwrite system in which the direction of the magnetic field applied is reversed for each rotation of a medium by using two or three laser beams, so that recording and erasure are conducted for each track of the medium. This system has the shortcoming that the apparatus for this system has a complicated structure and is expensive. The two-layer system is directed to the accomplishment of overwriting by use of a magneto-optical recording medium with a recording layer which is divided into two layers. This system is disclosed, for instance, in Japanese Laid-Open Patent Application 62-175948. In the two-layer system disclosed in this reference, a magneto-optical recording medium consisting of a recording layer consisting of TbFe and an auxiliary layer consisting of TbFeCo is employed. Overwriting is accomplished by initializing the recording medium and then applying thereto laser beams with different powers and an external magnetic field. More specifically, in this system, the magnetization of the auxiliary layer is oriented in one direction with application of a magnetic field for initialization of about 4 KOe prior to a recording step, the temperature T of the recording medium is raised above the Curie temperature Tc.sub.2 of the auxiliary layer, that is, T&gt;Tc.sub.2, with application of a high power laser beam, a magnetic field for recording with a magnetization direction opposite to that of the magnetic field for the initialization is applied, thereby reversing the magnetization of the auxiliary layer, and the magnetization is transferred to the recording layer when the recording medium is cooled, whereby recording is accomplished. Furthermore, the recorded information can be erased by raising the temperature T of the recording medium to a temperature between Tc.sub.1 and Tc.sub.2, that is, Tc.sub.1 &lt;T&lt;Tc.sub.2, where Tc.sub.1 is the Curie temperature of the recording layer, with application of a low power laser beam thereto and transferring the magnetization direction of the auxiliary layer to the recording layer. However, while this system has the merit that recording is performed at high speed, the problems arise that a high laser power is necessary for this system when writing and a large magnetic field must be applied for the initialization of the recording layer.
Several overwrite systems for magneto-optical recording such as the above-mentioned system have proposed. However, each system has its own strong points and weak points. It is said that many breakthroughs are necessary for developing those systems to such a level as to be usable in practice.
On the other hand, overwrite systems utilizing demagnetizing field have also been proposed, such as a system reported in Han-Ping D. Shieh & Mark H. Kryder; Appl. Phys. Lett. 49,473 (1986). This system is a kind of light modulation system. This system has a single-layer structure, and high capability of writing with application of a low power laser beam compared with the two-layer system, and no magnetic-field for initialization is necessary. However, in such an overwrite system utilizing demagnetizing field, the formation and the erasure of the magnetic domain are controlled by utilizing the differences in demagnetizing field energy, domain wall energy and Zeeman energy at a domain wall position, which are caused by the differences of the conditions for the application of a laser beam, so that the properties of the magnetic layer and the conditions for the application of a laser beam are dispersed. As a result, this system has the shortcoming that magnetic domains in which signal cannot be erased completely, and unnecessary magnetic domains are apt to be formed in the recording medium.